It is known to obtain pulses from the main winding of a brushless d-c motor when the motor operates. The frequency of the pulses is proportional to the instantaneous rotor speed and can therefore be used for measuring or controlling the motor speed. However, the frequency is relatively low, to wit, p.times..omega..sub.mech, wherein p is the number of pole pairs and .omega..sub.mech is the angular velocity of the rotor per second. The main, or power current flowing in the winding of the motor affects the pulses in their relative phase position with respect to each other and relative to the position of the rotor. A speed control employing these pulses for comparing nominal or command values and actual values is undesirably affected and continuous oscillations of the rotor may result.